Sugar beet has been cultivated for thousands of years as a sweets source, but its potential as a source of sugar was not discovered until the 18th century. The cultivated sugar beet (Beta vulgaris ssp. vulgaris L.) is a biennial plant belonging to the Chenopodiaceae. Its usual life cycle is completed in two years. In the first year a leaf rosette and a large succulent root is developed, which serves as a reserve for energy in the form of sucrose. For this reason it is farmed as an annual. In the second year shoot elongation (bolting) and flower formation starts after a period of low temperature and flowers and seeds are produced. If there happens to be prolonged cool periods in the first year, the seed stalk can already sprout. This genetically determined thermal induction leads to a phenomenon called bolting. Cropping the beet for sugar extraction cuts the biennial cycle in half, whilst the sucrose is at its peak.
Traditionally, there exist two methods for cropping sugar beet, spring and autumn cropping, which are practiced in the southern, milder climate or in northern latitudes, respectively. Both rely on varieties with different degrees of natural bolting resistance. Bolting resistance influences temperature, length and irradiation limits tolerable for seed stalk induction and is a key trait in sugar beet breeding. To allow for complete control of bolting and flowering, by either blocking vernalization, devernalizing vernalized plants or suppressing flower or viable seed production, would allow the sugar beet crop to be sown in autumn in northern latitudes without the risk of bolting and flowering in the following season. This shift from a spring into a winter crop would permit growers to drill their crop in autumn and to harvest the next summer. A comparison of winter cultivars to spring cultivars in crops like wheat and oilseed rape has shown that winter cultivars consistently yield higher than spring crops. The result would be an improvement of the economic viability and profitability of the crop. A further advantage would be the possibility to combine the growing of spring and winter crops, which would result in an extension of the harvest campaign by starting two to three months earlier, thus allowing for the improved capitalization on investments in equipment and infrastructure necessary for sugar beet harvesting, transport and processing. This extension of the sugar beet processing campaign would address one demand of the sugar industry.
The cold-induced vernalization as an obligate part of the complete sugar beet life cycle induces bolting of the plants. Vernalization and its effect on biennial sugar beet have been described in detail (e.g., JAGGARD et al., 1983). Sugar beet responds to temperatures between 3 and 12° C. Several weeks of 3 to 12° C. are required for the beet to start bolting. The ITB Bolting Model shows that in France, vernalization occurs up to 90 days (13 weeks) after drilling. Seventeen days of 7° C. is the critical number during these 90 days to initiate bolting. The likelihood of bolting is increased in relationship to the number of days on which the maximum temperature does not exceed 12° C. This can lead to loss of yield when the early sowing method is applied, as 1% bolters in a crop have been estimated to reduce sugar yield by 0.4-0.7%.
The knowledge about the vernalization response as the most important factor of flower induction has led to the development of bolting resistant sugar beet varieties which were made available to sugar beet farmers. However, cultivation of present sugar beet in central Europe over winter (i.e., as winter crop) to meet the demand of the sugar industry is currently not possible. Even with the cultivation of the higher yielding winter beet there are still major problems due to bolting incidents. The vernalization inducted by the exposure to the cold temperatures during winter still results in bolting and yield loss as currently no plants or methods are available for predictably delaying sugar beet vernalization.
For the foregoing reasons, it is highly desirable to develop non-bolting winter beet in which bolting resistance is engineered, e.g. by transgenic means, in order to modulate the vernalization response to confer resistance against or significantly delay of bolting after cold-induction. There is thus a need for nucleotide sequences of genes involved in the vernalization response and transgenic means making use of said gene sequences for modulating the vernalization response of sugar beet. The present invention now provides such nucleotide sequences and such transgenic means.
Functional analysis in the model organism Arabidopsis thaliana has distinguished four distinct flowering pathways (LEVY and DEAN, 1998). These four pathways can be assigned to environmental stimuli, such as photoperiodic and vernalization promotion pathways, or inherent developmental signals, e.g. autonomous promotion and floral repression pathways. In some species the timing of flowering is primarily influenced by environmental factors, such as photoperiod, light quality/quantity, vernalization and water or nutrient availability. Other species are influenced less by exogenous signals and rely more on endogenous ones, such as plant size or number of nodes.
One locus of interest is the FLOWERING LOCUS T (FT) discovered in naturally occurring late-flowering ecotypes of Arabidopsis (KOORNEEF et al., 1991). FT is a small protein of 23 KD and is homologous to phosphatidylethanolamine-binding proteins, which are also called RAF kinase inhibitor proteins (KARDAILSKY et al., 1999; KOBAYASHI et al., 1999).
The present inventors now have identified orthologues of the FT genes in sugar beet. Engineering of the expression of these genes has lead to a modulation of the vernalization response in sugar beet and to sugar beet plants in which the vernalization response is delayed or suppressed.